Configurable Prometheus path

[tobbbles]

Hi team. Currently the prometheus query path is hardcoded. This makes it difficult to utilize tools such as Mimir as this introduces /prometheus prefix to all API paths, and I imagine it's possible that other users are running their Prometheus server(s) behind a proxy with varying paths.

I'd propose a way to make this configurable, with the current value (/api/v1/query_range) as a default.

[mhratson]

please review #2120 as the solution